1. Field of the Invention
The present invention relates to a semiconductor device, and for example, to a via and an interconnect structure in a semiconductor device having a multilayer interconnect structure.
2. Description of the Related Art
Recently, instead of Al (aluminum), used in the prior art, Cu (copper) has been used as a material for interconnects in a semiconductor device of a multilayer interconnect structure. The use of copper is based on demands for a reduction in interconnect resistance and the conduction of a large current through interconnects.
Copper interconnects have a high EM (Electro Migration) resistance but do not sufficiently resist SM (Stress Migration). The reason is as follows. First, as shown in FIG. 16, an interconnect layer 101 and a via 102 are formed by burying copper in a via hole (connection hole) and an interconnect trench, respectively. If copper is buried using a CVD (Chemical Vapor Deposition) method, a sputtering method, or the like, crystal grains of the copper deposited in the via hole and interconnect trench through their respective positions (their side, bottom surface, or the like) have different crystal orientations. Thus, a grain boundary is formed in the interface where these crystal grains meet. The state of the crystal grains is unstable in an area close to the grain boundary 103.
When the semiconductor device is thermally treated, the unstable crystal grains attempt to migrate to a stable position owing to heat applied to the interconnect. As a result, as shown in FIG. 17, a void 104 may be formed close to the area in which the grain boundary 103 was formed. In particular, the crystal grains in the via 102 tend to be smaller than the crystal grains in the interconnect layer 101. This means that a large number of unstable grains are present in the via 102. Defects are likely to result from SM caused by the void 104.
To avoid SM defects caused by the migration of crystal grains, several measures can be taken. First, it has been proposed that, as a material for the interconnect and the via, metal different from copper be added to copper. As a result, the migration of the crystal grains can be hindered to suppress the formation of a void. However, when copper is mixed with a different type of metal, the low-resistance characteristic of copper becomes insignificant to increase interconnect resistance.
Another possible technique suppresses the migration of crystal grains by improving the adhesion between copper and a barrier metal arranged on a side wall of the via hole. However, this technique is effective on crystal grains present close to the barrier metal but is ineffective on the interface, where crystal grains are most unstable. That is, it is unavoidable that a void is formed in the grain boundary.